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Strong coupling between quantum emitters and photons in a one-dimensional waveguide is a key element for waveguide quantum electrodynamics (QED), a regime of great interest for universal quantum computing and communication. The basic ingredient of waveguide QED, a single two-level system (TLS) in a waveguide, can behave as a mirror whose transparency depends on the frequency and power of the incoming radiation.

In this work we present our experimental results on the system consisting of two superconducting qubits embedded in a copper waveguide realizing an analogue of a Fabry-Perot interferometer. Two external coils provide control over the flux threading the qubits, thus allowing us to individually tune their transition frequencies and to change the effective distance between the mirrors it in situ.

By exploiting the quantum properties of the mirrors we achieve new functionalities of the interferometer. Most notably, when the TLSs are asymmetrically detuned with respect to the frequency of the incident radiation, the system exhibits previously unobserved non-reciprocal behavior and operate as a microscopic light diode.